From the last couple I've done:
9 lbs. DME increased volume by ~1 gal.
12 lbs. DME increased volume by ~1.25 gal.
I haven't done any measurements with LME. How much volume is 5.5 lbs. LME?
EDIT: looking at various sellers, 6 lbs. LME comes in a half gallon bottle. 5.5 lbs. LME in that case would be about 0.46 gal.
4.4 lbs. DME would increase volume by ~0.46 - 0.48 gal.
Basically, it looks like you don't need to add water when converting from LME to DME.